1. Technical Field
This invention relates to digital images including video. More specifically, this invention relates to a dequantizer used for decoding of digital images and video compressed by a DCT-based (discrete cosine transform) transform coding, such as JPEG, MPEG and H.26x .
2. Description of the Related Art
Emergence of Internet video as well as high definition television, not to mention the literally millions of digital images currently available on Internet, has been fueling the recent surge of interest in compression of digital images. In particular, international standards such as JPEG, MPEG and H.26x, for compression of digital images and video have received much attention due to the fact that they are open standards for any developers.
Image (and video) compression is implemented in an encoder for encoding images using a quantization matrix. The decoder then is able to decode the image, also by using a quantization matrix. In the conventional art, the enconder and the decoder use the same quantization matrix. Recently, however, algorithms have been suggested, where the encoder uses one quantization matrix and the decoder uses a different quantization matrix. The new quantization matrix, used during decoding, is computed (by the encoder) using an approach similar to Miller's least squares solution as disclosed in Miller K., “Least squares methods for ill-posed problems with a prescribed bound”, SIAM J. Math. Anal., vol. 1, pp. 52-74, Febuary 1970 for image restoration applications.
Another solution is disclosed in Philips W., “Correction to 'JPEG dequantization array for regularized decompression”, IEEE Trans. on Image Proc., vol. 6, no. 6, pp. 883-888, 1997, which offers a dequantization scheme different from the standard method.
Konstantinides, et al. propose yet another technique for computing a modified quantization matrix for image sharpening applications directly in DCT domain. See Konstantinides K. Bhaskaran V. and Beretta G., “Image sharpening in the JPEG domain”, IEEE Trans. on Image Proc., vol. 8, no. 6, June 1999.
However, neither approaches guarantee that the dequantization process will map the quantized DCT coefficients to its value±(quantizer spacing/2) in DCT domain. As a result, a smooth recovery is not possible, especially for compressed images at a low bit-rate.
Therefore, there is a need for a new dequantizer that can recover the original image in a smooth manner.